As shown in Japanese Patent Application Laid-Open No. 2008-219063, a vehicle surroundings monitoring device configured to detect a solid object around a vehicle and to detect whether the solid object is stationary or moving has heretofore been disclosed. This vehicle surroundings monitoring device is configured to convert two images captured at different times into bird's-eye view images, and to align the two bird's-eye view images. Then, the vehicle surrounding monitoring device acquires a difference between the two images after the alignment and detects a different portion as a solid object. Further, the vehicle surroundings monitoring device detects the nearest point of the different portion to its own vehicle as a grounding point where the solid object is in contact with the ground, and detects whether the solid object is stationary or moving based on a positional change of the grounding point.
Here, it is known that the solid object is inclined when the captured image is converted into the bird's-eye view image. Due to such inclination, the vehicle surroundings monitoring device disclosed in Patent Literature 1 cannot achieve very high detection accuracy in detecting a travel distance of the solid object. Specifically, since the vehicle surroundings monitoring device disclosed in Patent Literature 1 detects the grounding point, the device calculates the travel distance based on a travel amount of the grounding point in order to obtain the travel distance. However, the travel distance thus calculated is not correct if a point detected as the grounding point before the travel and a point detected as the grounding point after the travel are not the same point of the solid object. To be more precise, there may be a case where the point detected as the grounding point before the travel is a lower end of a tire of another vehicle, whereas the point detected as the grounding point after the travel is a bumper thereof, for example. In this case, the travel distance is calculated based on not the same position of the vehicle and the travel distance thus calculated is incorrect.
An object of the present invention is to provide a travel distance detection device and a travel distance detection method which are capable of improving accuracy in calculating a travel distance.